The present invention relates to black magnetic iron oxide particles which are suitably used as black coloring pigments for paints, resins, printing inks, etc., because of a good blackness thereof, and as magnetic materials for binder-type carriers, and more particularly to black magnetic iron oxide particles which are capable of providing toners having a high image density even under high-temperature and high-humidity conditions, an improved keeping property of the image density and a less occurrence of fogging even under low-temperature and low-humidity conditions when used as black magnetic particles for magnetic toners.
Magnetite particles (“particles” are also occasionally referred to as a “particulate powder” in view of a shape or configuration thereof) are typical black pigments, and have been generally used for a long time as a colorant for paints, printing inks, cosmetics, rubber and resin compositions, etc.
In particular, the magnetite particles have been frequently used in one-component type magnetic toners in which composite particles prepared by mixing and dispersing black magnetic iron oxide particles such as magnetite particles in resins are employed as a developer.
In recent years, with the tendency of a high printing speed and a high image quality for laser beam printers or digital copying machines as well as the development of apparatuses capable of being operated under various environmental conditions, it has been strongly required to enhance properties of magnetic toners as a developer, in particular, provide toners capable of exhibiting a good keeping property of image density even under not only low-temperature and low-humidity conditions but also high-temperature and high-humidity conditions and a less occurrence of fogging.
In order to meet the above requirements for the magnetic toners, it is also strongly required that the black magnetic iron oxide particles used therein are further improved in properties thereof.
More specifically, in order to obtain toners which are excellent in environmental stability, in particular, keeping property of image density under high-temperature and high-humidity conditions, the black magnetic iron oxide particles used therein are required to have not only a sufficient resistance value as well as more excellent electric properties such as a high breakdown voltage, but also excellent environmental stability of these properties.
The reason therefor is due to the fact that upon forming a toner image, an image force as a resultant force of an electrostatic attraction force and a magnetic constraint force is exerted on toner particles when the toner particles fly towards a latent image formed on a photosensitive member, and an intensity of the image force is delicately controlled to attain a good balance between image density and fogging.
Namely, the toner particles having a high resistance value are improved in charging performance and, therefore, tend to readily fly towards the photosensitive member, resulting in a high image density. However, when the charging performance of the toner particles is too high, fogging tends to be caused. On the other hand, when the charging performance of the toner particles is too low, the fogging tends to be hardly caused, but it may be difficult to attain a sufficient image density.
In order to control the charging performance of the toner particles, there may be usually used a charge controlling agent. As the other means for controlling the charging performance of the toner particles, there is known the method of controlling an electric resistance value of magnetic iron oxide particles as a pigment component exposed to the surface of the respective toner particles. More specifically, when the electric resistance value of the magnetic iron oxide particles exposed to the surface of the respective toner particles is high, the toner particles tend to be readily charged. On the contrary, when the electric resistance value of the magnetic iron oxide particles exposed to the surface of the respective toner particles is low, an electrostatic charge on the surface of the charged toner particles is leaked through the magnetic iron oxide particles exposed to the surface of the respective toner particles upon stirring in a toner hopper, resulting in decreased charge amount of the toner particles.
These phenomena tend to become more remarkable under some environmental atmospheres to which the developing device is exposed, in particular, under high-temperature and high-humidity conditions or under low-temperature and low-humidity conditions. More specifically, in general, the charging performance of the toner tends to be lowered under high-temperature and high-humidity conditions, resulting in low image density, whereas the charging performance of the toner tends to be too high under low-temperature and low-humidity conditions, resulting in occurrence of fogging.
Therefore, when the black magnetic iron oxide particles are used as a pigment for the toner particles, well-controlled electric characteristics of the black magnetic iron oxide particles are very important to obtain images having a high image density without occurrence of fogging.
As to the electric resistance value of the black magnetic iron oxide particles, it is generally known that since magnetite exhibits electric characteristics of a semiconductor, a high electric resistance value thereof is realized by coating or attaching a high-resistance component (such as high-resistance oxides, hydroxides, dielectric organic substances, hydrophobic organic substances, etc.) on the surface of the respective black magnetic iron oxide particles by a dry or wet method.
Conventionally, it has been attempted to improve various properties of the black magnetic iron oxide particles by incorporating different kinds of elements other than iron thereinto and coating the surface thereof with an inorganic or organic substance.
For example, in Japanese Patent Application Laid-open (KOKAI) No. 5-213620(1993), there is described black magnetic iron oxide exhibiting a high electric resistance value by allowing a Si component to be contained therein or exposed onto the surface thereof. Also, in Japanese Patent Application Laid-open (KOKAI) No. 8-208236(1996), there is described black magnetic iron oxide coated with an oxide layer containing Fe and Zn.
Further, in Japanese Patent Application Laid-open (KOKAI) No. 2000-272924, there is described black magnetic iron oxide exhibiting a high electric resistance value which is coated with a hydrophobilizing agent.
Further, in Japanese Patent Application Laid-open (KOKAI) No. 2003-192350, there is described black magnetic iron oxide coated with a composite oxide layer containing an aluminum component and a magnesium component.
In addition, in Japanese Patent Application Laid-open (KOKAI) No. 2004-161551, there is described black magnetic iron oxide coated with a composite oxide layer containing Ti and Fe.
At present, it has been strongly required to provide black magnetic iron oxide particles exhibiting a high electric resistance in a high voltage range. However, the conventional black magnetic iron oxide particles have failed to satisfy these requirements.
That is, in the conventional techniques described in Japanese Patent Application Laid-open (KOKAI) Nos. 5-213620(1993), 8-208236(1996), 2000-272924, 2003-192350 and 2004-161551, the electric resistance value of the particles has been noticed, and studies on the electric resistance value of the particles have been made mainly in a low voltage range. However, the voltage of an electric field applied to the toner particles within a printer in which the toner particles are actually used, generally reaches the range of several hundred volts though it varies depending upon the kind of printer used.
In order to achieve a high image density and a good keeping property of the high image density, it is important that a pigment used in the toner exhibits a high electric resistance value, and the electric resistance value of the pigment is also kept high even in a high voltage range. More specifically, even though the pigment exhibits a high electric resistance value in a low voltage range, if the electric resistance value is low in an electric field actually used, an electrostatic charge present on the surface of the toner tends to leak out through the pigment exposed to the surface of the toner as a leak site, resulting in low charge amount on the toner and, therefore, considerable deterioration in image density.
Thus, all of the conventional techniques described in Japanese Patent Application Laid-open (KOKAI) Nos. 5-213620(1993), 8-208236(1996), 2000-272924, 2003-192350 and 2004-161551 are concerned with enhancement in electric resistance value in a low voltage range, and have therefore failed to satisfy the requirement of enhancing the electric resistance value in a high voltage range which has been strongly needed at the present time.